Durable airship hull and in situ airship hull repair

ABSTRACT

A system for airship hull reinforcement and in-situ repair includes a sensor for detecting a leak in the airship hull of a lighter-than-air airship and a repair mechanism inside lighter-than-air airship for dispensing repair material to seal the leak. A durable airship hull includes an inner gas barrier, an outer gas barrier, and a microlattice layer sandwiched between the inner gas barrier and the outer gas barrier.

RELATED DOCUMENTS

The present application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Application No. 61/563,187, by Stephen B. Heppe, filed onNov. 23, 2011, and entitled “Durable Airship Hull and in situ AirshipHull Repair,” which application is hereby incorporated by reference inits entirety.

BACKGROUND

Lighter-than-air airships maintain their buoyancy using lifting gascontained in a rigid or non-rigid hull. The lifting gas is typically alight gas such as hydrogen or helium. These lifting gasses can diffusethrough an undamaged hull and rapidly leak through macroscopic holes andtears in damaged hull sections. A macroscopic hole or tear (hereinaftergenerally discussed as a “hole”) can lead to rapid loss of lifting gas,loss of altitude, and potentially loss of the entire airship. Theproblem is particularly severe for very high altitude airships intendedfor stratospheric operation, where the hull material may be relativelylight-weight and therefore prone to damage. For example, JAXA reportedin 2008 regarding a flight experiment performed the previous year. A300,000 m³ balloon, with a hull comprised of Heptax, was successfullylaunched from Brazil on Nov. 20, 2007. However, it burst at an altitudeof 14.7 km. JAXA theorized that the balloon may have suffered damagearound the balloon spooler or collar. JAXA has also noted thatunexpected gusty wind conditions during balloon inflation could lead tosevere damage. Even if prompt loss of the airship is avoided, a smallhole will result in a significant loss of lifting gas over time, therebyleading to a loss of altitude and a relatively rapid termination of themission. A small hole can be caused, for example, by a naturalmicrometeorite or a man-made projectile such as a bullet or a piece ofman-made space debris falling back into the atmosphere, as well asongoing abrasion, excessive stress due to wind gusts, and other factors.

If a way could be found to minimize the likelihood of catastrophicdamage, and rapidly repair lightly-damaged hulls “in situ” (i.e., whilethe airship is still at altitude), the rapid loss of lifting gas couldbe prevented or halted and adverse consequences could be minimized.

This invention is directed to the problem of airship hull robustness andin situ airship hull repair.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate various examples of the principlesdescribed herein and are a part of the specification. The illustratedembodiments are merely examples and do not limit the scope of theclaims.

FIG. 1A illustrates an exploded view of a layered structure representinga cross section of an airship hull, according to one example ofprinciples described herein.

FIG. 1B shows a layered structure which sandwiches a microlatticeenclosed by gas barriers to form an airship hull, according to oneexample of principles described herein.

FIG. 1C shows an illustrative system for detecting leaks in a balloonand recovering lifting gas, according to one example of principlesdescribed herein,

FIG. 2A illustrates a roughly spherical airship with a central structurecomprising a rotating stage and an articulated camera, according to oneexample of principles described herein.

FIG. 2B illustrates a roughly spherical airship with a central structurecomprising a rotating stage, articulated arm, and camera, according toone example of principles described herein.

FIG. 2C shows a system for detecting leaks in an airship hull thatincludes a microlattice structure, according to one example ofprinciples described herein.

FIG. 2D is a simulated infrared image of cooling caused by lifting gasescaping through an airship hull, according to one example of principlesdescribed herein.

FIG. 3 illustrates a manipulator arm with a repair mechanism, accordingto one example of principles described herein.

FIG. 4A illustrates a portion of an airship hull with a hole in it,according to one example of principles described herein.

FIG. 4B shows a cross section of a hole in an airship repaired with apatch and injection of sealant, according to one embodiment ofprinciples described herein.

FIG. 5 is a flow chart of an illustrative method for in situ airshiprepair, according to one example of principles described herein.

FIG. 6 is a schematic view of an airship with a longitudinal passagewaythat also serves as part of the structure of the airship, and severalballonets for lifting gas, according to some of the principles describedherein.

FIG. 7 is a flowchart of an illustrative method for in situ airshiprepair in an airship with multiple ballonets, according to some of theprinciples described herein.

Throughout the drawings, identical reference numbers designate similar,but not necessarily identical, elements.

DETAILED DESCRIPTION

A stratospheric balloon or airship is generally designed with alight-weight hull so as to minimize overall vehicle size. For example,balloons intended for operation in the upper stratosphere may have ahull thickness less than 50 μm, weighing less than 100 g/m² of effectivehull surface area. Such balloons must be handled with care, and areeasily damaged on the ground as well as in flight. Further, the rate atwhich a lifting gas, such as hydrogen (H₂), leaks through a surface,such as a balloon hull, will depend on its material and construction aswell as operational factors such as the internal pressure of theballoon. Generally, efforts are made to limit the leakage rate throughcareful design. Such efforts are especially important for long-endurancemissions. U.S. Pat. No. 5,279,873, awarded to Oike, describes a materialthat exhibits a leakage rate for H₂ of 3 ml/m² over a 24 hour periodunder standard atmospheric pressure (101 kPa). If this is scaled to thecase of a large stratospheric airship with a surface area of 40,000 m²operating with an internal pressure of 520 Pa, the leakage rate would beon the order of 1 kg of lifting gas every 100 days. This must bereplenished for long-duration missions, in order to prevent a loss oflifting capacity. A reservoir of lifting gas can be used, but for verylong-duration missions, this reservoir may also require replenishment.

In the event that a hole occurs, lifting gas will leak out through thehole as well. The leakage rate will depend on the size of the hole, itsdischarge coefficient, the pressure differential between ballooninterior and external (ambient) conditions, and the density of the gas.One formula that can be used to estimate the leakage rate is

Q=C _(d) A ₀√{square root over (2(P ₁ −P ₂)/ρ)}

where Q is the volumetric leakage rate (discharge rate), C_(d) is thedischarge coefficient, A₀ is the aperture size, P₁ and P₂ are theinternal and external pressures, and ρ is the density of the gas. For apressurized hydrogen-filled balloon or airship designed for operation inthe upper stratosphere (e.g., 35-40 km), a reasonable value of P₁−P₂=100Pa, and ρ=0.5 g/m³. Also substituting C_(d)=0.6, and assuming a holewith A₀=1 cm² (10⁻⁴ m²), the volumetric leakage rate would be on theorder of 0.04 m³/sec. The mass leakage rate would be about 0.02 g/sec.If these parameters were held constant over time, a kilogram of liftinggas would escape in 50,000 seconds—less than a day, and therefore morethan 100 times faster than the nominal leakage through an undamaged40,000 m² hull satisfying the best-case performance reported in U.S.Pat. No. 5,279,873.

It may be appreciated that the illustrative parameters used here wouldnot remain constant—as lifting gas escapes, the balloon will descend andthe pressure difference will tend toward zero. Random fluctuations inthe hull (fluttering) will then tend to pump lifting gas out andatmospheric gases in. The balloon or airship will continue to descend asit loses lifting gas, and as its internal lifting volume becomes“polluted” with normal atmospheric gases that cannot provide buoyantlift. Clearly, even a small hole can have serious repercussions for ahigh-altitude long-endurance balloon or airship.

In the following description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present systems and methods. It will be apparent,however, to one skilled in the art that the present apparatus, systemsand methods may be practiced without these specific details. Referencein the specification to “an example” or similar language means that aparticular feature, structure, or characteristic described in connectionwith the example is included in at least that one example, but notnecessarily in other examples.

Because of the negative consequences of a damaged hull, there is a needfor hull materials that are resistant to damage, and furthermore thereis a need for hull materials that resist tearing and catastrophicfailure once a small hole occurs. In the event that a hole does occur, acapability to repair the hole in a timely manner, while the airship isaloft, would be beneficial. In order to repair a hole, three objectivesshould be achieved: a) identify the location of the hole; b) maneuver arepair mechanism to the location of the hole; and c) repair the hole. Inaddition to these objectives, it is also beneficial to purge theinterior lifting space of the balloon or airship of nitrogen and otheratmospheric gases that may have entered the balloon or airship throughthe hole, or even by diffusion through the undamaged hull. The threeinitial objectives noted above (identify location, maneuver repairmechanism, and actual repair) can be achieved through the use of sensorsand manipulators jointly adapted to identify the location of a hole(breach) in the hull, maneuver a repair mechanism to the vicinity of thehole, and repair it. In one embodiment the sensors, manipulators andrepair mechanism are controlled from a ground control station throughthe exercise of a command and control link, with data and feedback fromthe airship provided to the ground control station by way of a telemetrylink. In this embodiment, individual actions (such as e.g. camerapointing, zoom, exposure control, arm movement, repair mechanismalignment, repair) may be scripted and commanded by a human operator, orcollected into semi-automated procedures that can be initiated underhuman control. In another embodiment, the sensors, manipulators andrepair mechanism operate autonomously under stored program control,although sensor data and system status are optionally provided to theground control station as an adjunct to normal operations.

Strengthening the Hull

A high-order objective must be to avoid, or at least minimize thelikelihood of, a catastrophic hull failure that would cause immediateloss of the airship. A catastrophic hull failure can occur, for example,when a small hole rapidly expands or “tears”, thereby creating a largehole that allows a rapid release of lifting gas. Note that the originalhole might be quite small and operationally insignificant (at least inthe short term)—it is the expansion or tearing that leads to acatastrophic failure. In order to prevent such expansion or tearing, thehull can be built with “rip stop” features such as patterning of thebasic hull material (e.g., cross-hatched ridges), or high tensilestrength threads embedded in the hull material. The high tensilestrength threads can be natural or man-made fibers (e.g., fiberglass,plastic, metal, or even threads consisting of carbon nanotubes). Anotherapproach is to build the hull with a layered or sandwich structure asillustrated in FIG. 1A, which is an exploded view of a three-layerstructure comprising layers 102, 104 and 106. Layers 102 and 106 are gasbarriers such as Heptax or the material described in U.S. Pat. No.5,279,873 (to Oike) and noted above. By themselves, these layerssubstantially limit diffusion of lifting gas but may have insufficienttensile strength to strongly resist tearing. Layer 104, in contrast, isdesigned to act as a foundation for the gas barrier layers, provide hightensile strength, and strongly resist tearing. One structure that canbeneficially be used as layer 104 was recently disclosed by TobiasSchaedler and other researchers at HRL Laboratories and the CompositesCenter at the University of Southern California (the illustration oflayer 104 is taken from their media announcement and article in the Nov.18, 2011 issue of Science). The researchers describe the structure as anultralight metallic microlattice. It is composed of thin-walled tubes ofnickel (actually 93% nickel and 7% phosphorous by weight, according totheir disclosure) arranged in a lattice pattern as shown in FIGS. 1A and1B. The overall structure can have a density on the order of 10 mg/cm³to less than 1 mg/cm³ (counting the solid structure but not the air inthe pores) depending on the manufacturing technique and the thickness ofthe deposited metal. For example, the microlattice may have a density ofapproximately 0.9 mg/cm³. Such a microlattice, scaled up to a sheet 1meter square and 1 mm thick, would weigh roughly 1 gram. Recalling thata prior-art airship might rely on a hull material that weighs between 50and 100 g/m², it will be appreciated that an ultralight metallicmicrolattice, of the general type described by Schaedler et al, can beadded with only a small weight penalty. In one exemplary embodiment,instead of a 50 μm hull (gas barrier), an ultralight metallicmicrolattice 1 mm thick is bonded and sandwiched between two separategas barriers, each only 25 um thick. The microlattice represents atensile mat, or web, that tends to reduce fluttering and rippling of thehull surface, and also resists tearing. These characteristicssubstantially limit the likelihood of a catastrophic failure. The gasbarrier layers can be bonded to the microlattice with an adhesive, oralternatively by heat treatment (gently heating the gas barriers untilthey adhere to the microlattice).

Many other choices for layer thicknesses and hull design are feasible,beyond those described here, and any particular embodiment will betailored based on consideration of gas barrier and microlatticematerials and material characteristics available at the time of design,and mission requirements and parameters. For example, aerographite orother suitable lattice materials could be used. Aerographite is asynthetic microlattice made from a porous interconnected network ofcarbon microtube material. Samples of aerographite have been formed withdensities of 0.2 mg/cm³. Aerographite microlattice sustains extensiveelastic deformations due to its interconnected tubular structure. Asused in the specification and appended claims, the term “microlattice”refers to a wide range of materials, including porous materials thathave densities that are less than 10 mg/cm³.

Furthermore, the principles described herein are not limited to athree-layer sandwich structure. Other embodiments include a single gasbarrier bonded to a microlattice on only one side, or multiple gasbarriers and multiple microlattice layers arranged in an alternatingstructure (for example, three gas barrier layers and two microlatticelayers arranged in an alternating fashion). The different layers of thesame general type (i.e., gas barrier or microlattice) may be constructedof different materials, and have different thicknesses, densities, andother physical properties. Furthermore, the individual layers maythemselves be comprised of several “sublayers” one or more of which mayalso be considered a coating. For example, a gas barrier layer (asrepresented by 104, 106 in FIGS. 1A, 1B) may be comprised of an adhesivesublayer, a gas barrier sublayer, and an infrared reflection sublayeradapted to minimize heat loss from the airship.

An additional benefit of a sandwich structure comprising at least onemicrolattice layer and two gas barriers becomes apparent when oneconsiders the process by which lifting gas will diffuse through anundamaged hull. Lifting gas (e.g., hydrogen) will diffuse through theinner gas barrier into the thin shell, between the two gas barriers,represented by the microlattice. The gas in this thin shell will, ingeneral, be a mixture of lifting gas and normal atmospheric gases. Thepartial pressure of lifting gas, in this thin shell, will be less thanthe partial pressure of lifting gas in the interior of the balloon, butgreater than the partial pressure of lifting gas in the outsideatmosphere (where it is essentially zero). Thus, if nothing else isdone, the lifting gas will also diffuse through the outer gas barrierand be lost. The rates at which the lifting gas will diffuse, througheach of the (at least) two gas barriers, depends on the material andconstruction of the gas barriers and the difference in partial pressureson the opposite sides of each barrier. For an extremely long-endurancemission, any lifting gas lost to the outside atmosphere must ultimatelybe replenished from internal stores or external resupply.

In order to assemble an entire balloon or airship with hull materialbuilt in a sandwich structure as described above, individual “segments”or “gores” can be assembled with the gas barriers extended laterally andbonded together as illustrated in FIG. 1B. The gas barriers 102 and 106are bonded together into flanges 108, which can in turn be bonded toflanges from adjacent “segments” or “gores” assembled in a similarmanner. While the gas barriers are shown as separated from themicrolattice in FIG. 1B, in many embodiments the gas barriers will bebonded to the surfaces of the microlattice 104. This avoids snagging andtearing of the gas barriers on small imperfections of the microlattice.Each segment or gore can have variable cross-sectional width along itslength (for example, “pumpkin lobe” shaping), in order to allow propershaping of the balloon or airship as a whole. The flanges can bereinforced with additional material of the same or different type, andreinforcing structures can also be applied transverse to the strips orgores (i.e., spanning multiple gores).

FIG. 1C shows a system for detecting leaks in a balloon or airship andrecovering lifting gas. This figure shows a portion of the airship hullwhich includes an inner gas barrier 145, an outer gas barrier 150, and amicrolattice 155 sandwiched between the gas barriers. The thicknesses ofthese layers have been greatly enlarged for purposes of illustration. Asdiscussed above, an airship hull may be formed from a number of joinedsections 160,165. The sections are separate sealed compartments. Theairship may be formed from a number of these sections joined together.In an undamaged airship, there is no route for gas to pass between thesections except by permeation through the gas barriers. For simplicity,only two sections are illustrated in this figure, a left section 160 anda right section 165 (“left” and “right” being referenced to an observerlocated on the inside of the airship).

The system includes a vacuum pump 105 which is connected to each of thesections 160, 165. The output of the vacuum pump 105 is connected to agas separator 110 which separates the lifting gas from atmospheric gasor other contaminants. The separator may use a variety of systems forgas separation including membrane diffusion, centrifuge, adsorption,absorption, or distillation techniques. For example, membrane diffusionmay be performed using palladium, ceramic, or synthetic membranes. Thevacuum pump 105 and gas separator 110 are here shown as being locatedoutside the outer gas barrier 150, but could also be located inside theinner gas barrier 145.

The vacuum pump 105 pumps out gas from the sections. It will beappreciated that the pressure in the sections is already rather low (forthe case of a stratospheric system), the balloon is large, the sectionsare thin, and the microlattice structure itself represents a barrier tomovement of the gas through the lattice structure. Hence the vacuum pumpmay not evacuate the gas very quickly. Nevertheless, it will tend toevacuate the gas more quickly than it can diffuse through the outer gasbarrier (at least locally, near the point at which the vacuum pump isconnected to the thin shell volume). Multiple points of attachment(i.e., of the vacuum pump to the thin shell volume), located across thehull, can be used to more efficiently evacuate the mixture of gases fromthe thin shell volume. Because of the action of the vacuum pump, thepressure in the thin shell volume will tend to be lower than that of theballoon interior, and possibly even lower than the outside atmosphere.The difference in pressure will lead to a compressive force on themicrolattice, but this can be sustained by a microlattice designed withthis constraint in mind. Additionally, this compressive force can atleast partially compensate for tensile stresses experienced by themicrolattice and gas barriers.

The gas extracted by the vacuum pump from the thin shell sections can beseparated into its constituent parts to isolate the lifting gas (e.g.,hydrogen) from the ambient atmospheric gases (primarily nitrogen andoxygen). The gas separator 110 receives the output from the vacuum pumpand separates the lifting gas from the atmospheric gas. The hydrogen canbe re-introduced to the lifting volume of the balloon, and the nitrogenand oxygen can be vented to the outside atmosphere. In this way, theoverall rate at which lifting gas is lost to the outer atmosphere can besubstantially reduced.

Identifying Hole Location

Regardless of the hull construction type (three generic types weredescribed above: 1) a generally uniform hull material with no particularrip-stop features; 2) a hull material that incorporates rip-stopfeatures such as texturing or embedded fibers; and 3) a hull comprisinga sandwich structure with one or more layers designed as a gas barrier,and one or more layers designed as a tensile mat or web), small holesmay occasionally occur. When a hole occurs, it must be located andpatched in a timely manner (else the lifting gas will eventually belost). Two methods (among others) that can be productively employed toidentify a hole and its location are: a) photographic or videoobservation (e.g., with a camera) using ambient or artificial light; andb) active probing with a scanning laser such as a lidar system. Incertain cases, other methods may be used. For example, in the case of ahull sandwich structure comprising an electrically conductive materialsuch as nickel, electrical or electro-magnetic measurements (includingradar) may be used to identify and locate a hole that is large enough tohave damaged the microlattice structure (although such measurementsmight not identify a small hole in a gas barrier, where there was nodamage to the microlattice structure).

The system shown in FIG. 1C can be used as a leak detector. FIG. 1Cshows a hole in the right section 165 that has penetrated through boththe outer gas barrier 150 and the inner gas barrier 145. Escapinglifting gas 135 enters the section from the balloon interior 115 andcontaminating atmospheric gas 140 enters the section from the exterioratmosphere 120. Because the vacuum pump 105 is pumping gas out of theright section 165, a portion of the escaping lifting gas 135 and thecontaminating atmospheric gas 140 is captured and drawn into the vacuumpump 105. This reduces the contamination of the lifting gas in theballoon interior and minimizes the amount of escaping lifting gas 135.As the combination of escaping lifting gas 135 and contaminatingatmospheric gas 140 enters the vacuum pump 105 through a vacuum line, itis sensed by a sensor 146. A variety of sensors can be used to senseairflow into the vacuum pump, such as a mass flow sensor, a simplespectrometer or other suitable sensor. The information from the sensorscan be used to detect the amount and possibly the composition of gasentering the vacuum pump. The amount of gas indicates the level ofintegrity of the section to which the vacuum pump is attached. High gasflow rates indicate that the section may have been compromised while lowgas flow rates indicate a higher level of section integrity. The gasflow rates can be directly correlated to the size and/or location of arupture. For example, if there are two vacuum lines attached to thesection in different places, the general location of a leak along asection can be determined by comparing the mass flow rates in the twovacuum lines. In this example the leak is comparatively closer to afirst vacuum line than a second vacuum line. Consequently, the firstvacuum line will have a higher flow rate and the second vacuum line willhave a lower flow rate because of the greater distance the gasses haveto travel through the microlattice to reach the second vacuum port.

Gas composition in the vacuum line can be used to indicate whether theinner or outer gas barriers are ruptured (or both). For example, if thegas in the line is primarily hydrogen or helium, it can be assumed thatthe outer gas barrier is intact. If the gas composition in the vacuumline is primarily contaminating atmospheric gas, the outer gas barrierhas probably been ruptured, but the inner gas barrier is intact. In theexample shown in FIG. 1C, the sensor 146 detects a relatively largevolume of gas with a composition that is a mixture of atmospheric gasand lifting gas. Thus, it can be concluded that a rupture has occurredthat passes through both the inner and outer gas barriers in the rightsection 165. In contrast, relatively low gas volumes are received fromthe left section 160, indicating the left section has not beencompromised. The separator directs recovered lifting gas 125 into theballoon interior 115 and vents atmospheric gas 130 back out into theexterior atmosphere 120.

The schematic diagram shown in FIG. 1C is intended to show one exampleof principles described and is not limiting. A variety of otherconfigurations could be used. For example, the vacuum pump may be on theinterior of the balloon and/or may be connected to each section in twoor more positions.

FIG. 2A illustrates in schematic form a roughly spherical orpumpkin-lobed balloon (airship) 200 with a central structure 204 towhich a rotating stage 202 with an articulated still or video camera 206has been attached. By rotating the stage 202, and pointing the camera206 up and down, the entire hull surface can be examined from the centerof the balloon thereby identifying holes that are large enough to beseen from that location. An alternative embodiment dispenses with therotating stage and instead mounts a plurality of cameras with left-rightas well as up-down articulation capability in roughly the same location,such that their fields of regard overlap and jointly cover the entirehull. The camera(s) may have zoom capability, i.e. to allow moredetailed examination of the hull, and are not limited to visible light.Subsystems for power supply, pan/tilt (and optionally zoom), datahandling, command and control, and the like are assumed to be presentand are not further discussed. An auxiliary light source, either whitelight or wavelength(s) tailored to maximize contrast between the hulland a hole (where no hull material exists), may optionally be included.For balloons that include a microlattice layer, exposure of themicrolattice through a tear in the membrane could be sensed bydifferences in light reflection. The light source(s) may be fixed orarticulated, and may be separate from the camera(s) or integrated withthe camera(s).

Distinctive markings may optionally be placed on the hull tounambiguously identify the section of the hull contained in a camerafield of view, and also the relative hole location.

In an alternative embodiment, the camera(s) are replaced by a lidarsystem tailored to measure the range of the hull material from thesensor. Preferably, the hull and the lidar are designed or selectedtogether so that the hull material reflects a portion of the incidentlight, thereby enabling a range measurement at the sensor. Since a holewill not reflect the incident light from the lidar, hole locations canbe identified. The lidar can also be used to measure broad distortionsof the hull, potentially allowing preventive maintenance in the eventthat a weakened section is identified.

FIG. 2B illustrates an alternative embodiment 200 where a camera 206 ismounted on an articulated arm 208 that can bring it into close proximitywith the interior surface of the hull. The arm 208 articulates at therotating stage 202 to allow the camera 206 to be scanned up and down(i.e., substantially from “pole to pole”) within the balloon.Optionally, the camera 206 is also articulated at its point ofattachment to the arm 208, allowing up-down as well as right-left camerapointing. One or more tension lines can be used to support the free endof the articulated arm 208, and provide for additional degrees of motioncontrol. The combination of the rotating stage, articulated arm, andcamera, allows the hull to be examined in close proximity. Thisembodiment is expected to be able to identify smaller holes than theembodiment illustrated in FIG. 2A, but may require greater amounts oftime to completely examine an entire hull. This general concept can beextended to dirigible-shaped and other substantially non-sphericalairships by adapting the reach of the articulated arm, or providing fora plurality of such systems spaced apart within the airship (possiblywithin separate internal spaces or ballonets), or by mobilizing thesubsystem so that it can be repositioned within the airship in order tosupport reliable observation over the entirety of the hull.

FIG. 6 illustrates in schematic form a dirigible-shaped airship withouter hull 600 and multiple internal ballonets for lifting gas 610, 620,630, 640, 650, 660, and 670. Also shown is a longitudinal hollowpassageway 605 running the length of the airship, along its axis,passing through each of the ballonets. The cross-section of thepassageway may be circular, rectangular, or some other shape, and is notrequired to be uniform over the entire length of the airship (althoughit could be). In some embodiments, the passageway is supportedperiodically along its length by guy wires running between the ballonetsand out to the outer hull, which is assumed to represent a sufficientlyrigid support based on the differential between the internal pressure ofthe airship and the outside ambient atmospheric pressure. In someembodiments, the passageway is spaced away from the longitudinal axis ofthe airship for at least a portion of its length—for example, it may runalong the lower centerline or “keel” of the airship. The ballonets aresealed against the passageway so that each ballonet represents aseparate airtight space for lifting gas. In some examples, the space 607between the ballonets and between the ballonets and the outer hull canbe filled with lifting gas or with atmospheric gas. The lifting oratmospheric gas may be at the same or a different pressure than thelifting gas in the ballonets. In one example, the space between theballonets and the hull is filled with lifting gas that is pressurizedabove the ambient pressure and is substantially equal to the pressure inthe ballonets. Consequently, there is little, if any, structural load onthe membrane that makes up the ballonets during normal operation. Thus,the ballonet membrane can be designed to be substantially impervious togases, but may be a light-weight material that is not required to havesignificant structural strength. The ballonets prevent mixing of thelifting gas with the atmospheric gases contained in the space 607between ballonets, and between the ballonets and the outer hull. Alsoshown in FIG. 6 are a plurality of hatches 680 in the wall of the hollowpassageway 605, one hatch per ballonet, which are operative to open andclose. When closed, in a preferred embodiment, each such hatch wouldprovide an air-tight seal preventing the free transfer of lifting gasbetween the ballonet in which it is contained and the interior of thehollow passageway 605. This allows the individual ballonets to beselectively filled (pressurized) and unfilled (depressurized). Finally,FIG. 6 illustrates in schematic form a plurality of cameras or othersensors 690 (shown as a set of black dots), disposed in each of theballonets, for detecting holes in the ballonets as well as the outerhull proximate to the ballonet in which the camera or other sensor isdisposed. While only one camera or sensor is illustrated in eachballonet, the reader will understand that multiple such cameras orsensors could be disposed in each ballonet, and the schematicillustration could also represent fixed or articulated sensors asdiscussed above. In an embodiment where there are no internal divisionsor ballonets within the airship (or perhaps only a single largeballonet), a single camera or sensor can be mobilized along thepassageway 605 such that it can be repositioned within the airship inorder to avoid shallow look angles and support reliable observation overthe entirety of its length.

Many alternative mechanical arrangements can be used to allow formaneuvering of a camera or other sensor so that it can observesubstantially the entire inner surface of the balloon or airship.

In a system where the airship hull comprises a metallic microlattice,additional methods for identifying the presence and location of a holeare available. For example, a low-power radar operating in the EHFportion of the spectrum or above (i.e., above 30 GHz) could identify arange of small holes that represent damage to the microlattice structureand are large enough to be operationally significant.

As discussed above, in a system where the airship hull comprises amicrolattice and a vacuum pump is connected to the individual sectionsthrough one or more interfaces for extraction of gases from themicrolattice volumes, an increase in the flow of gas from a particularsection (or interface) can be used as a diagnostic to detect the likelypresence of a hole in that section (or near that interface).

FIG. 2C shows a system 212 for detecting leaks in an airship hull 222that includes a microlattice structure 155. The system 212 includes aninfrared (IR) source 214 and an IR camera 216. The IR source 214 is usedto heat up portions of the airship hull 222 while the IR camera 216images the heated portions. In some embodiments, the IR source may be aquartz lamp or other electrically operated heat lamp that comprises asource of radiant heat. In other embodiments, sunlight may be used asthe IR source. The right side of FIG. 2C shows a portion of the airshiphull 222 that includes an inner gas barrier 145, a microlattice 155 andan outer gas barrier 150. The airship hull 222 has a hole 228 throughwhich escaping lifting gas 135 passes. The hole 228 could have beenformed in a number of ways, including impact from an exterior object,material fatigue, or stress. In general, the balloon interior 115contains lifting gas at a higher pressure than the exterior atmosphere120. Consequently, the lifting gas passes through the airship hull 222and out the hole 228. Note that in implementations such as theembodiment illustrated in FIG. 6, where the outer hull is constructedwith a microlattice, a hole in the outer hull would primarily allow forthe escape of atmospheric gases from the internal space 607; however,the same event that caused the hole in the outer hull could have alsocaused a hole in one or more of the ballonets, and so the external holemay allow for the escape of a mixture of atmospheric gases as well aslifting gas). As the gas passes through the hole 228 it cools themicrolattice 155 that it passes over. This convective cooling effect isstrong enough to create significant temperature differences in themicrolattice 155 even for small holes. The IR camera 216 may have aminimum resolvable temperature difference on the order of tens ofmillikelvins. Consequently, the IR camera 216 can image temperaturedifferences caused by even small holes. Additionally, the IR camera 216may be equipped with an optical filter that is tuned toemission/absorption lines in a gas. For example, carbon dioxide has astrong absorption line at IR wavelengths between 4 and 5 microns. Thisabsorption can be used to directly image carbon dioxide leaking into anairship without internal atmospheric spaces (such as is illustratedschematically in FIGS. 2A and 2B), against the backdrop of the hull withits gas barrier(s) and its microlattice structure, using an IR camerawith an appropriate filter.

FIG. 2D is a simulated infrared image 218 of cooling caused by liftinggas (or atmospheric gases, or a combination) escaping from the airship.The region 226 is cooled by escaping gas and indicates the presence of ahole. After identifying a hole, the appropriate repair mechanisms can beused to patch the hole. Also visible in the image are reinforcingstrands 224 that have limited the extent of the hole.

Maneuvering a Repair Mechanism

As described above, FIG. 2B illustrates an articulated arm with a cameraor other sensor. This articulated arm can be adapted to carry a repairmechanism to a particular point of a balloon with a hole. In oneembodiment, the same arm that carries a camera system also carries arepair system able to affect repairs to the hull when the arm brings itinto close proximity to a hole. In another embodiment, a separate andindependent arm is used for the repair mechanism versus the camera orother sensor.

In an embodiment as illustrated in FIG. 6, each ballonet may contain adedicated maneuvering system that can support a repair mechanism, or aplurality of devices variously comprising one or several of a repairmechanism, camera, and other sensor(s). Alternatively, a singlemaneuverable and dexterous robotic device, adapted to carry one orseveral repair mechanisms and sensors, can use the passageway 605 withits hatches 680 to move between ballonets. Once located proximate to thehatch associated with a ballonet requiring repair, the hatch could beopened in order to allow the robotic device 697, or a portion of therobotic device, to maneuver itself out of the passageway and into theballonet. If the entire robotic device maneuvers itself out of thepassageway, it may perch itself on the outside of the passageway inorder to perform its work (deriving power either from an internalbattery or a power source on the outside of the passageway). This allowsthe hatch to be closed while the robotic device performs its work.However, in other embodiments only a portion of the robotic devicemaneuvers itself out of the passageway, and the robotic device remainssupported by the interior of the passageway or structures located in theinterior of the passageway. In such an embodiment, power can be providedby an internal battery, umbilical cable (which may also be adapted tocarry digital signals to and from the robotic device), or electrifiedrails. The ballonet with its open hatch will be fluidically connected tothe interior of the passageway, but the various ballonets can still beselectively filled (pressurized) and unfilled (depressurized) as long asthe other hatches remain closed.

Repairing the Hull

In one embodiment, the repair mechanism is an automated tape dispenserthat is adapted to apply at least one length of adhesive tape on theinside surface of the hull or ballonet such that it covers the hole inthe lifting gas enclosure subject to repair.

In another embodiment, as illustrated in FIG. 3, the repair mechanismcomprises an adhesive spray applicator 210 and a patch applicator 220both attached to a dexterous manipulator arm 230 such that they can beseparately brought to bear on the hole subject to repair. The dexterousmanipulator arm 230 is attached to a baseplate 240 which is itselfmounted on a manipulator arm (not shown) similar to that illustrated inFIG. 2B. In operation, the adhesive spray applicator 210 is firstbrought to bear and sprays an adhesive around the hole; then the patchapplicator 220 is brought to bear and a patch is applied over the holeand its surroundings, said patch being held in place by thepreviously-applied adhesive. Preferably, the same adhesive, or asecondary material that can serve as an adhesive caulk or gap-filler, isthen sprayed around the edges of the patch to seal the edges and preventleakage of lifting gas. If a secondary material is used, it may beapplied by the same spray applicator 210 (assuming the spray applicatoris adapted to accommodate multiple reservoirs of material), or adifferent spray applicator not shown.

Preferably, although it is not a strict limitation of the invention, theadhesive is pliable rather than brittle. It should be designed tooperate in the relevant environment (hydrogen atmosphere; expectedtemperature range) and “set” within a suitable amount of time—on theorder of minutes to a few hours. Total cure time can be longer. Alsopreferably, the tape or patch has rounded corners to minimize buildup ofstress in the hull or ballonet material adjacent to the tape or patch.

In order to ensure reliable placement of the repair tape or patch, andalso cater to a gas-tight seal, it is desirable to stabilize the repairmechanism relative to the hole subject to repair and also ensure thatthe hull material (in the case of a single-walled airship) or ballonetmaterial (in the case of an airship with ballonets) is stretched flatprior to placement of the tape or patch. One way to achieve this is toequip the repair mechanism with a set of dexterous fingers or grippers260, generally surrounding the tape dispenser or adhesive/patchdispenser, that are adapted to gently engage the hull or ballonetmaterial around the hole and spread the material until it is in tensionaround the hole, thereby ensuring a relatively flat and stable surfaceupon which to apply the tape or adhesive/patch combination. In FIG. 3,hardware to achieve this function is illustrated schematically as aplurality of manipulator arms 250 and grippers 260 located at thecorners of the baseplate 240 (although other arrangements are feasibleand within the scope of the invention). The manipulator arms 250 areadapted to allow the positioning of the grippers 260 within acommendable 3D volume referenced to the baseplate 240, thereby allowingthe grippers 260 to engage the hull or ballonet material at a pluralityof points and pull it taut in preparation for repair by the centralmanipulator arm 230 with its repair head (which, in this illustration,comprises a spray applicator 210 and patch applicator 220). In oneembodiment for a pumpkin-lobed balloon design, recognizing the relativedelicacy of the hull material, the grippers 260 have soft knobbed endpoints as illustrated in FIG. 3 and are adapted to engage thestrengthened longitudinal joints between the lobes of the balloon (thesections) as illustrated in FIG. 4. Here, gores 310, 312 and 314represent three adjacent sections of a pumpkin-lobed balloon wherein ahole 320 exists and has been identified in the middle of section 312. Inorder to spread the hull material around the hole and provide for arelatively flat and stable surface with a moderate degree of tension,grippers engage at the four points labeled A, B, C and D and tug gentlyaway from the hole, thereby placing the damaged section under tension.This stabilizes the location of the repair relative to the repairmechanism (thereby catering to precise placement), and provides arelatively flat hull surface for application of the repair tape (orpatch) such that an air-tight seal is more easily achieved. Grippers canbe designed generally along the lines of opposing fingers, forceps,pliers, or other suitable device.

The above discussion of repair techniques applies to single-volumeballoons without a microlattice hull structure, as well as repair of theinner gas barrier in single-volume balloons that comprise a microlatticehull structure. However, in such a system, it is also beneficial topatch the outer gas barrier or the microlattice structure itself. Thiswill prevent the escape of lifting gas contained within the thin shellvolume between the two (or more) gas barriers, and prevent the incursionof atmospheric gases into the shell volume (which over time couldoverload a vacuum pump designed to evacuate gases from that volume).Accordingly, in one embodiment, the repair mechanism of the inventiveconcept is further adapted to inject a quick-setting foam into themicrolattice structure (e.g., using a syringe), at several locationsspaced around the hole, so as to surround the hole with an annularbarrier that substantially prevents the flow of gas into and out of themicrolattice structure through the damaged section. At each point ofinjection, the foam will spread out and fill the interstices in themicrolattice structure between the inner and outer gasbarriers—incidentally also blocking the flow of gas through the tinyhole in the inner gas barrier at the point of injection. While the foambarrier could be built-up either before or after the repair of the innergas barrier, it is preferable to repair the inner gas barrier first(thereby eliminating the rapid outflow of gas).

FIG. 4B shows a cross section of an airship hull 222 with a hole 228repaired with a patch 415 and injection of sealant 405. As discussedabove, the patch 415 is first adhered to the inner gas barrier 145. Asyringe 400 pierces the patch 415 and/or the inner gas barrier 145 andinjects the sealant 405 into the microlattice 155. The injected sealant405 can be a variety of materials that cure under appropriateconditions. For example, the sealant 405 may be a two part epoxy that ismixed prior to injection, UV cured sealant, heat curing sealant,expanding foam, or other sealant. In some embodiments, the sealant mayfill the hole, bond to the patch, inner gas barrier, outer gas barrier,and microlattice.

Multiple Sets of Sensors

In one embodiment, two sets of sensors are used to identify holes in anairship hull or ballonet. A centrally-located sensor, or plurality ofsensors, as illustrated generally in FIG. 2A, is used to rapidly andrepeatedly scan the inner surface of the hull or ballonet and locatemoderate to large holes not visually obstructed by other internalstructures of the system. Separately, a close-up sensor on anarticulated arm (as illustrated generally in FIG. 2B), which optionallyalso hosts a repair mechanism, is used to slowly scan the inner surfaceof the hull or ballonet at higher resolution, thereby allowing thesystem to identify smaller holes that cannot be reliably seen orreliably identified by the centrally-located sensor or sensors. In anembodiment where the articulated arm hosts both a sensor and a repairmechanism, tasking for the arm and its end effectors should beprioritized such that the highest priority is to repair a large hole assoon as such a hole is identified, a secondary priority is to repair asmall hole, and a tertiary priority is to scan for small holes (i.e.,when the arm is not otherwise tasked with repair activities). In such anembodiment, the sensor is optionally used to provide for fineregistration and feedback control of the repair mechanism to ensureaccurate placement of the repair tape, adhesive/patch combination, orother repair materials (such as precise placement of a syringe forinjection of a foaming agent into a microlattice structure to which theinner gas barrier is bonded).

Preferably, the tasking is preemptive such that a high priority task(i.e., repairing a large hole) can interrupt a lower priority task(i.e., an ongoing repair of a smaller hole). However, in an alternativeembodiment an ongoing repair will be finished before the higher prioritytask is initiated.

Additional Considerations and Embodiments

A foaming agent may be added to the spray adhesive previously described,to create a thicker layer of adhesive around the hole, also minimizingthe likelihood of long unfilled “ripples” between the hull or ballonetmaterial and patch that could allow lifting gas to escape despite therepair. In a system that comprises an injectable foaming agent forblocking gas flow in a microlattice structure, the foaming agent addedto the spray adhesive could be the same or different from the foamingagent used to create a barrier to the flow of gas in the microlatticestructure.

In order to mitigate the effects of atmospheric gases which leak intothe balloon or ballonet(s) through macroscopic holes, as well as throughdiffusion, the airship can include a “scrubber” for nitrogen and oxygen.The scrubber can use any of several technologies to separate and isolatethe trace amounts of nitrogen and oxygen inside the balloon orballonet(s)—such as: cryogenic cooling (both nitrogen and oxygencondense at higher temperatures than hydrogen); centrifugal separation(nitrogen and oxygen molecules are substantially heavier than hydrogenmolecules); chemical scrubbing; and semi-permeable membranes. Afterseparation and isolation, the unwanted gases can be vented to theatmosphere. In a system comprising a hull with a microlattice structureand multiple gas barriers (at least an inner and outer), such a scrubbercan be used alone (i.e., operating only inside the primary liftingvolume of the balloon) or in conjunction with a vacuum system that alsoextracts gases from the thin shell regions between the gas barriers.

FIG. 5 is a flow chart of an illustrative method 500 for in situ airshiprepair. A leak location is identified from the interior of the airship(block 505). A variety of methods could be used to identify the leaklocation, including feedback from gas sensors on vacuum lines, IRcameras, visual inspection, lidar, radar or other suitable inspectiontechnique. Additionally, multiple inspection techniques can be usedtogether to more accurately and quickly identify a leak or otherstructural problem.

The repair mechanism is moved into position (block 510) and repairmaterial is dispensed from the repair mechanism to seal the leak (block515). As discussed above, the sealing may be performed using tape, foam,adhesive/patch, or other sealant. For example, adhesive may be dispensedover the portion of the inner gas barrier with the leak and a patchplaced onto the adhesive. In other embodiments, foam or other sealingmaterial can be injected into the microlattice. After repairing theidentified leak, the repair system again scans the interior of theairship for additional leaks.

In an airship comprising an outer hull and multiple ballonets forlifting gas, such as a dirigible-shaped airship using pressurizedambient air to maintain its shape and multiple internal ballonets forlifting gas spaced along its length, one or a plurality of “spareballonets” may be used in lieu of the repair techniques discussed above,or in an embodiment that can repair small holes but not large ones, tomitigate severe damage beyond the capability of the repair techniques toovercome. These spare ballonets would remain unfilled and stowed orfolded during normal operations prior to incurring damage to a filledballonet. After damage to a filled ballonet is sustained, asuitably-placed spare ballonet (i.e., close to the failed ballonet) isinflated from the remaining gas in the damaged ballonet, or apressurized reservoir of lifting gas, or an external source, or somecombination of these, thereby functionally replacing the damaged and nowdeflated ballonet. If the ballonets are designed to press against theouter hull when filled to an overpressure condition, the spare ballonet,or the spare ballonet and the remaining filled ballonets, can be sofilled, and thereby help to stabilize the outer hull until it can berepaired, or until the airship can be returned to the ground forservicing. Alternatively, the airship and its pump(s) for ambient airmay be able to maintain suitable operating pressure despite a hole inthe outer hull.

In an airship comprising an outer hull and multiple ballonets forlifting gas, such as a dirigible-shaped airship using pressurizedambient air to maintain its shape and multiple internal ballonets forlifting gas spaced along its length, where each ballonet contains asensing and repair mechanism as described earlier (or where a suitabledevice can be maneuvered into a damaged ballonet), an augmentedinventive method and associated apparatus can be used to mitigate damageto the outer hull as well as the ballonets. In the augmented method, theballonets are designed to press against the outer hull and adjacentballonets when fully inflated (note: as illustrated in FIG. 6, theballonets are not required to be fully inflated and pressed against theouter hull under normal operational conditions, although they may touchover a portion of their surfaces). Thus, when a ballonet is damaged, thefirst step is to inflate the damaged ballonet until it presses againstthe outer hull and adjacent ballonets. Note that a single damagingevent, such as a micrometeorite strike or bullet, may affect multipleballonets as well as the outer hull. Note further, that depending on thedetailed design of the airship and its ballonets, and the location ofthe damaged ballonet(s), some or all of the undamaged ballonets may alsoneed to be more fully inflated in order to ensure that the damagedballonet(s) press(es) against its neighbors. While inflating the one orseveral damaged ballonets may be counterintuitive, it provides astabilized surface to support repair by the sensing and repairmechanism(s) contained in the damaged ballonet(s). This approach alsoreduces the rate at which ambient pressurized air is lost through thehole(s) in the outer hull, and possibly enters the ballonets, since theflow of pressurized air between the ballonets and the outer hull isrestricted, and the damaged ballonets are pressurized to minimize theinflow of unwanted gasses. A sensing and repair method and apparatus aspreviously described can then be used to repair the damaged ballonet inwhich the apparatus resides. An augmented sensing and repair mechanismcan also repair the outer hull, if the ballonet in which it is locatedhappens to be pressed against a hole in the outer hull (note that thismay be the damaged ballonet or an undamaged ballonet). This is achievedby locating the hole in the outer hull using previously-describedtechniques (or other techniques known to those of skill in the art) andplacing a reinforcing patch, or several reinforcing patches, on theinside of the ballonet at the location of the hole in the outer hull.The augmented sensing and repair mechanism then uses a suction apparatusto pull the reinforced section of ballonet away from the outer hull,injects a foaming adhesive and sealer through the reinforced section tothe space between the ballonet and the outer hull, and then releases thesuction to allow the reinforced section to press once again against theouter hull, sandwiching the foaming adhesive and sealer between theoutside of the reinforced section of the ballonet and the inside of theouter hull. This bonds the reinforced section of ballonet to the outerhull, which seals and reinforces the outer hull at the location of thehole. Following repair of the ballonet(s) and outer hull, the ballonetscan be deflated to their normal operational size by pumping some of thelifting gas into other ballonets, venting some of the lifting gas to theatmosphere, or recompressing it into an onboard reservoir. Thissubstantially restores the desired buoyancy characteristics of theairship. The ballonet will tend to pull away from the outer hull, exceptwhere it is reinforced and bonded thereto. In some embodiments, liftinggas that is lost during the repair process can be resupplied fromanother airship.

The method just described can be adapted to an embodiment with a singlemaneuverable and dexterous robotic device that can maneuver from oneballonet to another as described previously (e.g., through a passagewaythat communicates with each of the ballonets). In such an embodiment,the single robotic device repairs each damaged ballonet as well as theone or several holes in the outer hull.

In a repair method that involves the over-inflation of one or moreballonets, the airship as a whole will tend to become more buoyant untilthe ballonets are subsequently deflated. This can be mitigated, in someembodiments, by adjusting dynamic lift associated with the airship, orby the application of vectored thrust, or by providing increased tensionon a tether extending to a lower airship or the ground. Further, in anairship that contains more than three ballonets, the number of ballonetsthat require full inflation can generally be limited to three or less(for a single damaged ballonet). This is achieved by first inflating theone or two ballonets that are adjacent to the damaged ballonet,essentially “locking” these one or two ballonets in place due tofriction between them and the outer hull. The damaged ballonet can thenbe inflated. If the excess lifting gas used to inflate these ballonetsis taken from ballonets that do not need to be inflated (i.e., they arefurther from the damaged section), the “supplying ballonets” can bedeflated while the “receiving ballonets” are inflated, and the overallbuoyancy of the airship can be minimally affected (although the trimcharacteristics may be affected, and of course a small amount of liftinggas is continually being lost until ballonet repairs are completed).

In a repair that results in the bonding of a ballonet to the outer hull,the subsequent deflation of the ballonet will result in the ballonetremaining locally attached to the outer hull. There will be some localstress on the ballonet, but this can be compensated (and the subsequentintegrity of the ballonet maintained) by ensuring a sufficiently largereinforced area.

In an embodiment such as is illustrated schematically in FIG. 6, thepassageway 605 can be adapted to serve as a plenum for distribution andredistribution of lifting gas. For example, with a single reservoir oflifting gas connected to the interior of the passageway through acontrollable valve, and separate controllable valves connecting theinterior of the passageway to each of the ballonets, the valves can beoperated to deliver lifting gas to a selected ballonet or severalballonets. Similarly, if a pump is provided to compress lifting gas fromthe passageway and store it in the reservoir, the valves and pump can beoperated to deflate a selected ballonet (or several ballonets) andcompress the lifting gas for storage in the reservoir. The hatches 680,illustrated in FIG. 6, represent a set of simple large-aperture valves;however, a more traditional set of valves that are separate from thehatches, and separately controllable, may also be used.

A single reservoir of lifting gas, and a single pump, operated asdescribed above, can sequentially inflate and/or deflate one or severalballonets in accordance with the repair methods described previously. Ifgreater speed is desired, simultaneous deflation of one or severalballonets, and inflation of one or several other ballonets, can beaccommodated if a pump and valve assembly is associated with eachballonet separately. This allows lifting gas to be pumped out of theballonet(s) intended to be deflated, while simultaneously delivering thelifting gas to the ballonet(s) intended to be inflated. In one suchembodiment, one-way pumps move lifting gas from the ballonet(s) intendedto be deflated, and the previously-described hatches are used as a setof simple large-aperture valves for the ballonet(s) intended to beinflated.

The flow chart of FIG. 7 summarizes an illustrative example of theprocess described above. A hole or holes in the ballonets/hull areidentified by location and size and the repair priority is determined(step 702). In some situations, location and size of a hole may bedirectly measured, such as imaging the hole with a camera or otheractive sensor. In other situations, the location and size of the holemay be indirectly measured, such as measuring a pressure loss in aballonet. Larger holes in the ballonets are given highest priority toprevent excessive loss of lifting gas and contamination of the remaininglifting gas in the ballonet with atmospheric gases. Other holes, such assmaller holes in the ballonets and holes in the hull are given lowerpriority. The location of the hole and operational events can alsoinfluence the prioritization of the repair. For example, a hole/tearthat compromises the physical integrity of the airship may be givenhigher priority than a hole that doesn't have structural implications.The highest priority hole is selected for the first repair operation,followed by holes with lower priority.

The ballonet with a hole to be repaired is stabilized, the hatch isopened, and the robot or other repair mechanism is moved into theballonet to make the repair (step 704). As discussed above, stabilizingthe ballonet may include a variety of actions, pressurizing and/orshifting the ballonet. Pressurizing the ballonet may be accomplished ina variety of ways, including introducing additional lifting gas into theballonet, introducing additional lifting gas into adjoining ballonets,decreasing the pressure in the hull (causing the ballonet to have apositive differential pressure), or other technique. Where a hole is inthe hull, the system may be partially self healing. For example, as thepressure in the hull drops, the ballonets will expand due to theincreasing pressure differential and may be pressed against the hole inthe hull. This may slow the leakage out of the hull. In other cases, aprojectile/meteorite may penetrate both the hull and the ballonet.Shifting the ballonet (for example, by inflating an adjacent ballonet)may serve to offset the hole in the ballonet from a corresponding holein the hull. Pressurizing the ballonet against the surface of the hullthen partially seals both holes until a repair can be performed.

The image to the right of block 705 illustrates a hole 695-1 in the hull600 and a corresponding hole 695-2 in the ballonet 630. In thisillustration, the ballonet 630 is shifted so that the holes are notaligned. This misalignment may be a result of the spacing between theballonet and the hull during normal operation of the airship or may be aresult of mechanical displacement of the ballonet. The ballonet has beenpressurized to press the ballonet membrane against the hull andpartially seal the holes. This prepares the ballonet and hull forpatching (step 705).

The robot or other mechanism repairs the hole in the ballonet membrane(step 710). This may be accomplished in a variety of ways, includingputting a reinforcing patch 632 over the hole and/or injection ofsealant.

In this example, a second patch is placed on the interior surface of theballonet in a location that corresponds to the hole in the hull (step715). This reinforces this portion of the ballonet membrane so that itcan serve as a patch for the hole in the hull 600. The repair robot 636or other mechanism then injects adhesive sealant 634 through the patch602 and over the hole in the hull (step 720). This seals the hole andadheres the ballonet membrane 630 over the hole 695-1. Theadhesive/sealant also fills the injection hole after the repairmechanism is with drawn. The adhesive/sealant cures and the repairmechanism can be moved to the next hole location (if any).

The preceding description has been presented only to illustrate anddescribe examples of the principles described. This description is notintended to be exhaustive or to limit these principles to any preciseform disclosed. Many modifications and variations are possible in lightof the above teaching.

What is claimed is:
 1. A system for airship hull reinforcement andin-situ repair comprising: a sensor for detecting a leak in the airshiphull of a lighter-than-air airship; and a repair mechanism inside theairship for dispensing repair material to seal the leak.
 2. The systemof claim 1, in which the hull comprises an inner gas barrier, a spacerlayer, and an outer gas barrier.
 3. The system of claim 2, in which thespacer layer is a microlattice having a density of less than 10milligrams per cubic centimeter.
 4. The system of claim 3, in which themicrolattice has a density of less than 1 milligram per cubiccentimeter.
 5. The system of claim 2, in which the repair materialcomprises a sealant injected into the spacer layer.
 6. The system ofclaim 1, in which the repair material comprises tape or an adhesivelyapplied patch.
 7. The system of claim 1, in which the sensor comprises avacuum pump and vacuum line gas sensor.
 8. The system of claim 7, inwhich the vacuum line gas sensor comprises a spectrometer.
 9. The systemof claim 1, in which the sensor comprises an infrared camera and radiantheat source.
 10. The system of claim 1, further comprising: a pluralityof ballonets disposed inside the airship hull; and a hollow passagewayconnecting the ballonets.
 11. The system of claim 10, further comprisinga plurality of hatches in the passageway providing access to an interiorof each of the said plurality of ballonets.
 12. The system of claim 11,in which the repair mechanism can access the interior of each of thesaid plurality of ballonets through the hatches.
 13. A durable airshiphull comprising: an inner gas barrier; an outer gas barrier; and amicrolattice layer sandwiched between the inner gas barrier and theouter gas barrier.
 14. The airship of claim 13, in which themicrolattice layer has a density of less than 10 milligrams per cubiccentimeters.
 15. The airship of claim 13, in which the microlatticelayer comprises a lattice of metal tubes.
 16. The airship of claim 15,in which the metal tubes comprise a nickel alloy.
 17. The airship ofclaim 13, in which the inner gas barrier has a thickness of less than 50microns, the outer gas barrier has a thickness of less than 50 microns,and the microlattice layer has a thickness of less than 2 millimeters.18. A method for in situ airship hull repair comprising: identifying aleak in the airship hull using sensors located in the interior of anairship; moving a repair mechanism into a position in proximity to theleak; and dispensing repair material from the repair mechanism to sealthe leak.
 19. The method of claim 18, in which dispensing the repairmaterial comprises dispensing tape from the repair mechanism onto aninner gas barrier of the airship hull.
 20. The method of claim 18, inwhich dispensing repair material comprises: placing a patch over theleak in the airship hull; and injecting sealant into a spacer layer inthe airship hull.
 21. The method of claim 18, in which identifying theleak in the airship hull comprises: pumping gas out of a spacer layer inthe airship hull; and measuring gas extracted from the spacer layer. 22.The method of claim 21, further comprising: sensing differentconstituent gasses in the gas extracted from the spacer layer;determining if the leak comprises atmospheric gas that passed through anouter gas barrier into the spacer layer; and determining if the leakcomprises lifting gas that passed through an inner gas barrier into thespacer layer.
 23. The method of claim 18, in which the airship comprisesat least one inner ballonet disposed within the hull, the method furthercomprising: pressurizing the ballonet; and moving the repair mechanisminside the ballonet to the location of the leak.
 24. The method of claim23, further comprising: placing a reinforcing patch on an inner surfaceof the ballonet such that the reinforcing patch is over the hole in thehull; and injecting adhesive sealant between the ballonet and hull suchthat the leak in the hull is sealed.